At a construction site, two workers are moving different piles of loose gravel. Worker A is using a Bobcat and Worker B is using a Caterpillar. Worker A has to move 60 tons of gravel at a rate of 0.35 tons per hour and Worker B has to move 180 tons of gravel at a rate of 0.85 tons per hour. After how many hours will both workers have the same amount of gravel left to move?